Depth of focus (or focal range) computation devices are known, such as for example, as described in Austrian Pat. No. 286.776, which on the basis of the adjustments or settings of the parameters which determine the depth of focus, calculate the value of the depth of focus, and produce a signal upon not reaching of a minimum value (which is defined or determined by means of a presetter) or of a predetermined position.
These type of depth of focus control devices have the disadvantage that the depth of focus must be calculated, which is most extremely expensive and complicated with a mechanical as well as an electronic type of design.